In a raster laser display, one or more beams of laser light are scanned across a screen at high speed to create a video display. Typically, a pair of mechanically actuated mirrors sweep the laser beam(s) horizontally across the screen in a series of vertically displaced rows to produce a raster scan similar to a television set. Beams of red, green and blue can be combined on the screen to form a full color display. The various colored beams may be combined at the point where they meet the screen or, alternately, may be combined earlier and transmitted to the screen as a single combined beam.
As the beam(s) are scanned across the screen, video information is fed to a modulator which modulates the laser beams to create the desired display. The modulator receives a stream of video information comprising individual packets of video data, each pocket corresponding to a dot of the display (individual packets hereinafter referred to as pixel data packets). The video display actually comprises thousands of dots (termed pixels hereinafter) arranged in rows and columns. Each pixel is discreetly illuminated by the laser in sequentially time order. The pixels are generated at a very high rate (typically over 1 MHz). The persistence of vision of the observer causes the display to appear illuminated with an entire video image. The scanning of the beam by the mechanically actuated mirrors and the provision of the stream of video data to the modulator are synchronized such that a pixel data packet corresponding to a particular pixel of the display reaches the modulator at a time when the actuated mirrors cause the beam(s) to hit the screen at the corresponding pixel position.
One known method of modulating a laser beam is by use of acousto-optic modulators positioned within the path of the beam(s). In a simple form, an acousto optic modulator directs the beam(s) either towards the screen (actually towards the actuated mirrors which then scan the beam across the screen in a series of subsequent, vertically displaced, horizontal lines) or towards a light absorbing substance. A typical acousto-optic modulator comprises an acousto optic medium positioned within the path of the laser light beam(s). The pixel data packets are converted to an RF acoustic signal by a piezoelectric crystal and introduced from the crystal into the acousto optic medium. In the absence of an acoustic signal propagating through the acousto optic medium, the beam passes through the medium undeflected. However, when sound propagates through the medium, the incident laser beam is deflected at a specified angle dependent upon the wavelength of the sound. In its simplest form, an acousto optic modulator utilizes a single frequency of sound. In the absence of the sound, the incident beam travels straight through the modulator towards a light absorbing substance. However, in the presence of sound, the beam is deflected towards the mechanically actuated mirror system which causes the beam to appear on the screen. The modulator(s) is positioned between the laser light source and the actuated mirror system. One or more of the beams are directed towards the screen by the modulator(s) when the pixel information packet indicates that that pixel of the screen is to be illuminated. Otherwise, the beams are directed towards the light absorbing substance when the pixel information packet indicates that the corresponding pixel position on the screen is to remain dark.
Continuous wave laser light sources suitable in power and wavelength for use in laser projectors tend to be extremely inefficient. Typically, for a continuous wave laser, i.e., a laser with a continuous beam of light, only about 3-10% of the light produced is released. The remaining light stays in the cavity of the light source to continue the light amplification process. The Coupler Transmissance of a laser is the ratio of the amount of light energy which is released from the source to the total amount of light energy produced in the source. Coupler transmission is given as a percentage. The Coupler Transmissance, however, is not necessarily an accurate figure. It is very difficult to actually measure the instantaneous power within a laser cavity since any sampling process in the laser cavity will change the cavity characteristics. Further, it is now believed that the relationship between the Coupler Transmissance and the ratio between internal and external light power may not be linear and that there may be excess optical power in the cavity in comparison to what the Coupler Transmissance would predict.
Beam modulation, whether acousto optic in nature or otherwise, is another cause of wasted light energy. When the modulator directs the beam away from the screen and towards the light absorbing substance, that light energy is not recovered. Typically, in a laser raster display, the display will have a very high proportion of dark content; typically on the order of 80-90%. Thus, in such a display, 80-90% of the light which is released from the laser light source is never used.
Therefore, it is an object of the present invention to provide an acousto-optic modulator in which significantly less light energy is wasted than in the prior art.
It is a further object of the present invention to provide a laser light system for a raster display having increased efficiency.
It is yet another object of the present invention to provide a laser light system for a raster display having an acousto optic modulator which returns the light energy which is not displayed on the screen to the laser light source.
It is yet one more object of the present invention to provide a laser light source having an acousto optic modulator within the laser cavity.
It is a further object of the present invention to provide an improved laser light source.
It is yet another object of the present invention to provide an improved raster laser display system.
It is one more object of the present invention to provide a laser light system for a raster laser display having an acousto-optic modulator which returns the light energy not displayed on the screen to the lasing medium for further amplification.